Field of the Invention
The present invention is directed to a composition for producing a ceramic dielectric and a method of making a ceramic dielectric from such a composition, wherein the composition can be sintered at a low temperature, has a high nonloaded Q value (hereinafter referred to as Q value) in a microwave bandwidth region, and is capable of being sintered simultaneously with a high conductivity conductor, such as silver or copper. The composition sintered at a lower temperature according to the present invention can be used, for example, in the production of ceramic dielectrics used in the construction of multilayer microwave resonators and filters.
Description of the Prior Art
The increase in the magnitude of communication information transmitted in recent years has promoted the rapid development of various communication system utilizing microwave bandwidth regions, such as cellular telephones, satellite communication and satellite broadcasting, as well as the development of various microwave dielectric materials to accommodate the industry""s needs. The ceramic dielectrics used in the microwave bandwidth region exhibit characteristics that the Q value tends to reduce as the working frequency is increased. They are, furthermore, required to have the following characteristics:
(1) a high Q value in the microwave bandwidth region,
(2) a high specific dielectric constant (er), and
(3) a small absolute value for the temperature coefficient (xcfx84f) of the resonancy frequency.
Various kinds of compositions having the foregoing characteristics have been developed. Known compositions having a high Q value include, for example, Ba(Mg1/3Ta2/3)O3 and Ba (Zn1/3Ta2/3)O3. Known compositions having a high specific dielectric constant include, for example, a BaO.TiO2.RE2O3 (in which RE represents a rare earth element), each of which is used as a resonator, a filter, or the like.
It has been recently proposed to employ a multilayer type dielectric resonator or filter using a conductor as an internal electrode to create multilayer ceramic materials utilized in a high frequency bandwidth region, such as a microwave, since it is necessary to form a high conductivity metallic conductor onto the surface of the composition. This is accomplished by sintering the conductor between ceramic layers. Noble metals, such as platinum or palladium, which are expensive but resistant to high temperature, are employed in the production of existing ceramic dielectrics in which the sintering temperature exceeds 1000xc2x0 C., thus resulting in increased production costs. It is extremely advantageous if a composition is sinterable at a low temperature, for example, about, 900xc2x0 C. This reduces the production costs since an inexpensive metal material, such as silver or copper, can be used as the conductor. Accordingly, the industry has demanded the development of a ceramic dielectric produced by a composition that can be sintered at a temperature of about 900xc2x0 C.
Generally, sintering compositions at low temperatures involves such methods as: (a) adding glass frit as a sintering aid; (b) utilizing finely atomized submicron size starting materials; or (c) utilizing a chemical process, such as the sol-gel method. However, since the usual compositions have low reactivity with glass, they suffer from the added difficulty of forming compositions at high densities creating the additional problem of drastically reducing the Q value.
It is an object of the present invention to overcome the foregoing problems and provide a ceramic dielectric created by sintering a composition at a low temperature of about 900xc2x0 C. Therefore, the compositions can be simultaneously sintered with a high conductivity conductor, such as silver or copper, to produce a ceramic dielectric having a high Q value in the microwave bandwidth and a small temperature coefficient of resonance frequency. A reduction in production costs is realized.
The foregoing object can be attained in accordance with the first aspect of the present invention by providing a ceramic dielectric by sintering a composition at a low temperature, when the composition comprises of a calcined powder, which contains from 0.1 to 20 parts by weight of ZaO, from 0.1 to 10 parts by weight of Ta2O5 and from 0.1 to 1 parts by weight of MnO2, based on 100 parts by weight of a composition represented by: BaO.xTiO2 (in which 3.0xe2x89xa6xc3x97xe2x89xa65.7). 0.1 to 20 parts by weight of a glass powder (based on 100 parts by weight of the calcined powder), having a transition point of lower than 450xc2x0 C., are added to the calcined powder and then sintered.
In accordance with a second aspect of the present invention the calcined powder comprises of at least one of the following: less than 1 part by weight of WO3, not more than 15 parts by weight of SnO2, not more than 15 parts by weight of MgO, not more than 10 parts by weight of SrO, and not more than 5 parts by weight of ZrO2 in combination with BaO.xTiO2 based on 100 parts by weight of the composition represented by BaO.xTiO2 (in which 3.0xe2x89xa6xc3x97xe2x89xa65.7).
Furthermore, according to the present invention, the product of the Q value and the resonance frequency of the composition may not be less than 2000 GHz.
Also, a ceramic dielectric can be obtained in accordance with a third aspect of the present invention by a process comprising the steps of combining starting materials to form a composition containing from 0.1 to 20 parts by weight of ZnO from 0.1 to 10 parts by weight of Ta2O5 and from 0.1 to 1 parts by weight of MnO2, based on 100 parts by weight of a composition represented by: BaO.xTiO2(in which 3.0xe2x89xa6xc3x97xe2x89xa65.7), calcining the composition of a temperature from 900xc2x0 to 1200xc2x0 C. to prepare the calcination product, then pulverizing the calcination product, admixing 0.1 to 20 parts by weight of a glass powder having a transition point of not higher then 450xc2x0 C., to 100 parts by weight of the resultant calcinated powder, molding the mixture into a predetermined shape and then sintering the same at a temperature from 850xc2x0 C. to 1000xc2x0 C.
Moreover, a ceramic dielectric can be attained in accordance with a fourth aspect of the present invention by a process, wherein the composition is further blended with at least one of the following: less than 1 part by weight of WO3, not more than 15 parts by weight of SuO2, not more than 15 parts by weight of MgO, not more than 10 parts by weight of SrO and not more than 5 parts by weight of ZrO2 as the starting material(s), based on 100 parts by weight of a composition represented by: BaO.xTiO2 (in which 3.0xe2x89xa6xc3x97xe2x89xa65.7).
The composition according to the present invention has a high Q value in a microwave bandwidth region, a small temperature coefficient of resonance frequency, and as extremely high density (low water absorption). According to the process steps for producing the ceramic dielectric according to the present invention, a composition having both excellent density and dielectric characteristics can also be produced. This is accomplished by adding a specified amount of glass, having a predetermined transition point, to a specific ceramic raw material composition, formulated to effectuate sintering at a relatively low temperature. Therefore, a composition excellent both in density and dielectric characteristics can be obtained, thereby promoting sintering at a lower temperature without being problematic, even in the case where the total amount of the glass powder used is small, by using a glass powder containing a predetermined total amount of PbO or a predetermined amount of PbO and R2O (R represents an alkali metal element) and further using a calcined powder and a glass powder each of a predetermined average grain size.
Applicants have made various studies of BaO.xTIO2 compositions capable of being sintered at a low temperature to obtain a composition having a high Q value while maintaining xcfx84f within a practical characteristic range. The present invention, which is premised on this discovery, has overcome the reduction in the density of the sintering composition caused by the addition of the glass by adding a small amount of a specific glass powder having a predetermined transition point, which overcomes the reduction in the density of the composition, to the calcined powder composition, containing a specific amount of a predetermined metal oxide of the Ba.TiO2 system, to the composition and then sintering them.
In the composition represented by BaO.xTiO2, x is preferably within a range from 3.0 to 5.5, and ideally from 3.5 to 4.5. If x is less than 3.0 or more than 5.7, the Q value of the composition is lowered. Furthermore, ZnO and Ta2O5 are added to enable sintering at a low temperature even if the total amount of blending glass is small. If the total amount of ZnO or Ta2O5 is less than 0.1 parts by weight, low temperature sintering is difficult. If ZnO exceeds 20 parts by weight or Ta2O5 exceeds 10 parts by weight, the Q value is lowered. The total amount of ZnO is preferably within a range of 1 to 12 parts by weight, and ideally within a range of 1 to 10 parts by weight, while the total amount of Ta2O5 is preferably within a range of 0.3 to 8 parts by weight, and ideally with a range of 0.3 to 5 parts by weight. If the total amounts for both ZnO and Ta2O5 are within the abovementioned preferred ranges, sintering can be attained easily at a low temperature without lowering the Q value. Additionally, MnO2 can be added to the composition to further improve sinterability. If MaO2 is less than 0.1 parts by weight the density is insufficient, whereas the Q value and er are reduced if it exceeds 1 part by weight. If the total amount of MnO2 is within a range of 0.1 to 0.8 parts by weight, and ideally within a range of 0.2 to 0.5 parts by weight, a composition of high density can be obtained without lowering the dielectric characteristic.
In a second aspect of the present invention, at least one of the oxides (WO3, SnO2, MgO, SrO and ZrO2) other than (ZrO, Ta2O5; and MnO2) may optionally be added. WO3 is added for improving the Q value and, if the total amount exceeds 1 part by weight, the Q value is lowered somewhat.
SnO2 is added for shifting the value of xcfx84f to the negative side and, if the total amount exceeds 15 parts by weight, the Q value and er are markedly lowered. The total amount of WO3 is preferably within a range of 0.5 to 1 part by weight and the total amount of SnO2 is preferably within a range of 1 to 12 parts by weight and, ideally within a range of 3 to 10 parts by weight. If the total amounts of the oxides are within the preferred ranges, a composition having excellent performance characteristics, e.g., a high Q and er value, can be obtained.
MgO is added for controlling the value of xcfx84f. Its addition decreases the absolute value of xcfx84f. If the total amount of MgO exceeds 15 parts by weight, the Q value and er are greatly lowered, SrO is added to increase Er and if the total amount exceeds 10 parts by weight, the Q value is lowered and xcfx84f is out of a practical range. Furthermore, ZrO2 is added for shifting xcfx84f negatively and if the total amount exceeds 5 parts by weight the Q value is lowered. To attain a dielectric ceramic having a high Q value and er and a practical value for xcfx84f, the total amount of MgO should preferably be within a range of 1 to 10 parts by weight, and ideally within a range of 3 to 8 parts by weight. The total amount of SrO is preferably within a range of 0.5 to 10 parts by weight, and ideally within a range of a 1 to 5 parts by weight. The total amount of ZrO2 is preferably within a range from 0.5 to 4 parts by weight, and ideally within a range of 1 to 3 parts by weight.
Glass is added for obtaining a dense sintering composition is low temperature sintering and if the transition point exceeds 450xc2x0 C., a sufficiently dense sintering product cannot be obtained, particularly, if the added total amount is relatively small. If the total, amount added is less than 0.1 parts by weight, in spite of the fact that the transition point of the glass employed is not higher than 450xc2x0 C., it is difficult to increase the density of the composition for low temperature sintering. On the other hand, if the total amount added exceeds 20 parts by eight, the Q-value is lowered remarkably, degrading the performance to such an extent that is virtually impossible to make measurements of the dielectric characteristics is a microwave bandwidth region. To obtain a composition with a sufficiently high density without reduction of the Q value and while maintaining excellent dielectric characteristics, the glass transition point is preferably within a range of 350xc2x0 to 450xc2x0 C., and ideally within a range of 370xc2x0 to 430xc2x0 C. The total amount of glass added is preferably within a range of 3 to 20 parts by weight, and ideally within a range of 5 to 15 parts by weight.
In a fifth aspect of the present invention, glass containing 5 to 60 mol % of PbO, based on the total amount of the glass, is added to further promote sintering of the composition at a low temperature. If the total amount of PbO is less than 5 mol %, the effect of promoting sintering is low, whereas the Q value of the resultant composition is lowered if it exceeds 60 mol %. The total amount of PbO is preferably within a range of 5 to 60 mol %, and ideally within a range of 10 to 50 mol %.
In a sixth aspect of the present invention, glass ideally containing 0.01 to 5 mol % of an alkali metal element oxide in addition to PbO is added to further improve sinterability and promote sintering at a low temperature even if the additional amount of glass is relatively small. If the amount of alkali metal oxide added is less than 0.01 mol %, the sintering promotion effect is small, whereas the Q value is lowered if the amount exceeds 5 mol %. The total amount of the oxide of the alkali metal element is preferably within a range of 0.01 to 5 mol %, and ideally within a range of 0.1 to 3 mol %.
In seventh to ninth aspects of the present invention, the average grain sizes of the calcined powder and the glass powder greatly influence the density of the sintering product. If the average grain size of the calcined powder is out of the range of 1 to 3 xcexcm, the density of the resultant sintering product is undesirably lowered. It is not preferable for the average grain size of the glass powder to be outside of the range of 0.1 to 1.5 xcexcm and particularly less than 0.1 xcexcm, since uniform dispersion and mixing with the calcined powder is difficult. If the average grain size of the glass powder exceeds 1.5 xcexcm, densification of the composition is undesirably insufficient. In a seventh aspect of the invention, the average grain size of the calcined powder should be within a range of 1 to 7 xcexcm, preferably within a range of 1 to 4 xcexcm, and ideally within a range of 1.5 to 2.5 xcexcm. In an eighth aspect of the invention, the average grain size of the glass powder should be within a range of 0.2 to 7 xcexcm, preferably within a range of 0.2 to 4 xcexcm, and ideally within a range of 0.5 to 1.5 xcexcm. Also, in a ninth aspect of the invention, the average grain size of the glass powder is preferably smaller than the average grain size of the calcined powder, and preferably within a range of 0.1 to 3 xcexcm, and ideally within a range of 0.3 to 2 xcexcm. By employing calcined and the glass powders having grain sizes within the foregoing ranges the powders can be easily dispersed and mixed homogenously and a composition of high density can be obtained.
In the tenth aspect of the present invention, if the calcination temperature is lower than 900xc2x0 C., the Q value is reduced, whereas if it exceeds 1200xc2x0 C., xcfx84F increases to a positive region and is outside a practical range. The calcination temperature is preferably within a range of about 900xc2x0 to 1200xc2x0 C. The calcination time is preferably within the range of about 2 to 10 hours, and ideally within a range of about 4 to 8 hours, although no particular restriction is implied. If the sintering temperature is lower than 850xc2x0 C., densifying is difficult, whereas if the sintering temperature exceeds 1000xc2x0 C., the Q value is reduced and xcfx84f is outside of a practical range. In addition, simultaneously sintering with a conductor material such as silver or copper becomes difficult. A preferable range for the sintering temperature is 850xc2x0 to 950xc2x0 C. A dielectric ceramic material sintered at a low temperature having a high Q value and a small xcfx84f and of high density can be obtained within this temperature range. Although there is no particular restrictions for the sintering time, it is preferably about 30 min to 5 hours, and ideally about 1 to 3 hours.
In the present invention, a composition sinterable at a low temperature with minimal degradation of the dielectric characteristic, having a high density and excellent dielectric characteristics can be obtained by adding a small amount of a specific glass ingredient having a low transition point to a calcined powder of the BaO.TiO2 composition.
Also, if the reactivity with the ceramic raw material is not sufficient by the use of low transition point glass only, glass containing a predetermined amount of PbO may be used. Additionally, is the present invention, glass also containing an oxide of an alkali metal element together, which is usually considered undesirable because of reduction of the Q value, may be used, whereby the sinterability at a low temperature can be improved further by a smaller amount of such glass. The reduction in the dielectric characteristic is lessened since the amount of the particular glass used is small.
The reader""s attention is specifically directed to the reaction between the ceramic raw material and glass in the present invention, when the calcined powder is sintered with the addition of the glass powder. The calcined powder and a liquefied glass react to form a liquid reaction product promoting sintering. Particularly, in the case of using glass containing PbO, consideration is given to the fact that the viscosity of the molten glass is reduced sufficiently at a sintering temperature of about 900xc2x0 C., and sintering proceeds by chemical reaction with the calcined powder ingredient, thereby enabling densification at a low temperature of about 900xc2x0 C.
The crystal phase of the composition obtained with the addition of glass is different from that obtained without the addition of glass. Furthermore, it is taken into consideration that the combination of the ceramic raw material of BaO.TiO2 and the glass of PbO-alkali metal element oxide provides a remarkable sintering promotion effect because both materials have a high reactivity to each other and forms a specific eutectic liquid composition between them.